The present invention relates to a creep prevention apparatus for a car equipped with an automatic transmission.
If a transmission lever is kept set to a drive position (advance position) during the stop of a car equipped with an automatic transmission, the dragging torque of a torque convertor causes so-called "creep phenomenon" in which the car moves forward against the will of a driver of the car. Since this creep force applies a braking force to the engine which is idling, a throttle opening must be kept open to such a level as to compensate for this braking force, in order to keep the number of revolution of the engine at a reference value. For this reason, fuel consumption becomes naturally greater as much. This is one of the main reasons why the car equipped with an automatic transmission has high fuel consumption.
The assignee of the present invention proposed previously an improved apparatus in which the pressure of an advance clutch is bypassed to an oil tank upon detecting the idling of a car so that the power transmission between the engine and driving wheels is cut off and the load to the engine is made to the same level as that of a car equipped with a manual transmission. However, the following problem will occur in an automatic transmission in which speed change gear trains (e.g. a first speed gear train) at the time of idling are also used at the time of engine brake. Although the prior art apparatus does not cause any problem at all so long as all the systems operate normally, the engine brake at the first speed gear train is hardly effective if the systems are kept under the creep prevention state for some reason or other. In a car of the type in which a shift position is provided so as to make the engine brake effective at this first speed gear train, therefore, the driver of the car necessarily relies upon a foot brake.
The possible cases in which the problem described above occurs are as follows. The first is the case in which a sensor system is out of order and judges the normal operation as idling. The second possible case is that an actuator unit is out of order and keeps the creep prevention state, although the sensor system and the judgement system are normal. Among the second case, it is highly likely that the operation of an oil pressure change-over valve as the final actuator becomes unsmooth due to any dust mixed in the oil, and the change-over valve can not return to the inoperative position if the force of a return spring is weak.
The problem of the kind described above does not occur in a car in which the shift position to make the engine brake effective at the first speed gear train is not provided. In a car of the type in which the engine brake is applied at the second speed gear train, however, the gear ratio of the second speed gear train can not be reduced freely in order to secure sufficient brake performance, and this becomes a critical problem in improving the fuel consumption.